1. Field of the Invention
The field of the invention is that of digital mobile radio systems. The invention applies in particular to cellular mobile radio systems such as systems conforming to the GSM (Global System for Mobile communications) standard, for example.
More particularly still, the invention concerns the exchange of digital signals, whether they constitute data or speech, in time-division multiple access (TDMA) time-division multiplex systems.
2. Description of the Prior Art
The TDMA technique divides time into frames of fixed and predetermined duration, the frames being in turn divided into time slots. Each call is associated with one or more time slots.
Thus a frame comprises N time slots that can correspond to N calls. Each receiver is able to extract the time slots addressed to it in order to reconstitute the source signal. In this way N calls can be transmitted in the same frequency band.
Where data communications are concerned, mobile radio systems like the GSM system conventionally provide two services, corresponding to two different, levels of quality. Thus, for transmission of data, the GSM system provides a first data communication mode called the full rate mode, in which a time slot is transmitted in each frame, and a second transmission mode called the half rate mode, in which the data signal is transmitted in one time slot only every two frames, on average.
In this second mode, the resource allocated to a call is halved compared to the first mode. This frees up resources for other calls.
This halving of the total bit rate of the call naturally requires modification of the channel coding used, to retain the same user bit rate, in other words, the half rate mode corresponds to channel coding with half the yield of that of the full rate mode. In the GSM system, the two modes respectively correspond to raw bit rates of 11.4 kbit/s and 22.8 kbit/s.
Consequently, the efficacy of half rate channel coding is less than that of full rate channel coding. For this reason the half rate mode can be used only when transmission conditions are good and/or the transmission quality required is average, in other words when a relatively high bit error rate can be tolerated. If the transmission channel is subject to interference and/or the data requires a higher transmission quality (i.e. a lower bit error rate) the full rate mode must be used.
According to the GSM standard, a transmission mode is chosen at the time the call is set up and is retained throughout the call. This technique has two drawbacks:
if the service in question requires a transmission quality corresponding to the use of the half rate mode under normal conditions of operability and if the half rate mode is adopted, should the channel then be subjected to a higher level of interference, exceeding the operability limit of the system (set at a C/I value of approximately 9 dB), the call in progress is suddenly cut off; under difficult coverage conditions a value of C/I below 9 dB may be encountered; PA1 if the service in question requires a transmission quality corresponding to the use of the full rate mode under normal conditions of operability and if the full rate mode is adopted, should the channel subsequently be subject to a lower degree of interference, the channel coding employed is of higher quality than is required; the transmission channel is therefore unnecessarily occupied in alternate frames (causing unnecessary interference in neighboring cells). PA1 to increase the number of calls in the multiplex; PA1 to reduce the transmission time (in the case of transmitting data); PA1 . . PA1 the analysis of transmission conditions, which were previously viewed from one end only (generally the base transceiver station); it was therefore possible to regard as "good" a channel that was "bad" in the opposite direction, and vice versa; the invention presupposes feedback of quality information (in at least one direction) before a decision is taken; PA1 the selection of the coding mode, which is selective for each transmission direction; and PA1 the encoding mode used, since the invention enables selective action at the source coding and/or channel coding level. PA1 n bidirectional transmission modes (known as primary modes) corresponding to the use of the same coding mode in both directions; PA1 n(n-1) bidirectional transmission modes (known as secondary modes) corresponding to coding modes in both directions. Among these n(n-1) modes there may exist situations in which the raw bit rate is different and therefore corresponds to different required minimal resources in the two transmission directions (to be more precise, there are modified transmission modes of two types: modes with the same raw bit rate in both directions, and modes with asymmetric raw bit rates). If the system requires symmetrical resource allocation, the resource corresponding to the highest raw bit rate is allocated. The resource allocated to at least one of said transmission directions is therefore greater than the resource required to transmit the information coded in the corresponding coding mode, and said coded information is divided between a fraction of the time slots corresponding to said allocated resource. PA1 either they do not carry any signal, which reduces interference with neighboring cells; if there is no transmission in a cell, the latter does not cause any interference in its neighbors, PA1 or they are allocated to the transmission of asynchronous data. PA1 a first (full rate) mode in which said data is transmitted at the rate of one time slot every signal frame, and PA1 a second (half rate) mode in which data is transmitted at the rate of one time slot every two signal frames. PA1 transmitting information coded in a first coding mode at the rate of one time slot every signal frame (full rate) in a first transmission direction, and PA1 transmitting information coded in a second coding mode at the rate of one time slot every two signal frames (half rate) in a second transmission direction, PA1 the bit error rate (BER) of the received signal, PA1 the power of the received signal, PA1 the distance between the mobile station and the base transceiver station, PA1 an estimate of the impulse response of the transmission channel, PA1 the time alignment, PA1 the signal to noise ratio, PA1 the signal to interference ratio (C/I). PA1 a required level of quality for the call in progress, PA1 a required level of quality for at least one transmission direction and for the call in progress, PA1 a type of service conveyed by said call, PA1 the traffic load. PA1 means for determining at least a first indication representative of transmission quality in the mobile station to base transceiver station direction, PA1 means for receiving a second indication representative of transmission quality in the base transceiver station to mobile station direction, PA1 means for modifying the coding mode and/or the transmission mode in each transmission direction in accordance with said first and second indications, and PA1 means for transmitting to said mobile station information representative of the coding and/or transmission modes selected. PA1 means for determining at least one indication representative of transmission quality in the base transceiver station to mobile station direction, PA1 means for transmitting said indication to said base transceiver station, and PA1 means for receiving an indication representative of the coding and/or transmission modes selected.
In mobile radio systems these problems are major problems since the transmission channel changes continually with the movement of the mobile station and the movement and the activity of the sources of interference, etc. As a result, the full rate mode is usually chosen, for safety, and this leads to high and often unnecessary consumption of the transmission resource.
There are also two configurations in the case of speech signals (full rate mode and half rate mode), which correspond to the use of different speech encoders (source coding) and different channel encoders, the two pairs of encoders (source and channel) providing respective raw bit rates of 22.8 kbit/s (full rate) and 11.4 kbit/s (half rate).
Problems similar to those described above for data are also encountered with speech.
A major objective of mobile radio system designers is to limit the quantity of data transmitted, for a number of reasons and in particular:
To this end, U.S. patent application U.S. Pat. No. 5,327,576 proposes modification of the mode of transmission used, during a call, on the basis of the measured bit error rate.
To be more precise, in the method described in the above application, the base transceiver station (the station managing all calls with mobiles in a given cell) measures the bit error rate of a given call and selects one or other of the transmission modes according to the measured error rate.
This technique improves transmission resource use. It has a number of drawbacks, however, that the novel technical approach of the invention clearly highlights. In particular, it is based on an analysis of the transmission channel as seen from the base transceiver station only (or from the mobile station only), which leads to non-optimum resource use, as will emerge below.
One object of the invention is to overcome these various drawbacks of the prior art.
To be more precise, one object of the invention is to provide a method of adaptation of the air interface (essentially corresponding to layers 1 (physical) and 2 (link) of the ISO model) in a mobile radio system that minimizes the occupancy of transmission channels by reducing the quantity of resource allocated to a call on average and by limiting interference induced by a call in neighboring cells.
This object of limiting interference is crucial in cellular mobile radio systems in particular. In these systems, the same frequency band is allocated to several geographically dispersed cells. Although the distribution of the cells is defined to maximize the distance between them, it is by no means rare for the signals of a given cell to suffer interference from those of other cells using the same band to a degree that is above an acceptable limit for the system.
In a cellular system, a maximum interference level enabling the specified transmission quality to be provided is usually fixed. An object of the invention is therefore to provide a method of the above kind whereby the specified transmission quality continues to be provided if the interference exceeds this maximum level.
An object of the invention is therefore to provide a method of the above kind in which the untimely cutting off of calls is reduced.
In other words, an object of the invention is to expand the range of operability of the system, in particular under difficult transmission conditions.
Another essential and primordial object of the invention is to provide a method of the above kind whereby the number of calls can be increased. In other words, an object of the invention is to reduce, on average, the resource used to transmit a service in order to increase the number of users in the system, i.e. the number of calls per cell.
In one particular embodiment of the invention, a secondary object of the invention is to provide a method of the above kind for transmitting asynchronous data easily and as fast as possible, in particular when it is not possible to free up the same resource in both communication directions.
Another object of the invention is to provide a method whereby the network infrastructure is simplified. In the conventional way, network planning must offer acceptable operability (C/I.apprxeq.9 dB in the GSM system) everywhere (or over a certain portion of the area of each cell), which in some cases constitutes a very serious constraint.
An object of the invention is therefore to remove this constraint by widening the operability range (beyond 9 dB, for example in the case of the GSM system) to allow more efficient planning by reducing the number of sites.
Another object of the invention is to provide a method of the above kind that is equally applicable to speech signals and to data signals.